PLATE XXIII.
Fig. 113.—Elongated Liquid Crystals of Dibenzal Benzidine, showing Double Refraction and Dichroism.
Fig. 114.—Spherical Liquid Crystals of Para-azoxy-anisol, showing Interference Colours under Crossed Nicols as the Effect of Compression.
Fig. 114 represents the effect of compression on para-azoxy-anisol, and demonstrates very clearly the distribution of the interference colours due to double refraction.
We are thus face to face in these remarkable experiments with some new facts concerning the nature of crystals. For we pass here into the borderland between ordinary liquids—singly refractive and structureless, in which the molecules are rolling over each other with every possible orientation—and solid true crystals possessing homogeneous structure, and the basis of which is a space-lattice arrangement of the chemical molecules, determinative of the system of symmetry displayed. In this wonderful borderland we certainly have had revealed to us, by the genius and persistency of Lehmann, liquids which possess many of the attributes of crystals, such as definite orientation of the ultimate particles, double refraction, and optic axes. These are undoubtedly solid facts which require to be faced.
Whether Lehmann’s theory is to be accepted in full can only be decided after much more investigation by several independent investigators. We are now becoming familiar with the phenomena, as they have naturally excited immense interest in all scientific circles, and demonstrations of many of the experiments of Lehmann have been given in this country by Dr. Miers, Prof. Pope, and others, and particularly by Messrs Zeiss, with their new high temperature microscope, a description of the use of which for the projection of liquid crystals on the screen will presently be given. Prof. Lehmann himself has described the phenomena so clearly and fully that it is quite easy for others to repeat his experiments, and doubtless time would often be much better spent in doing so than in criticising points of theory without observing the phenomena at first hand. It frequently happens, in the inevitable march of scientific progress, that striking new facts, such for instance as the discovery of the composite nature of the chemical atom, are apt to cause either alarm, even panic, as to cherished theories, or else unreasoning scepticism. The happy mean between these two modes of receiving such facts, the open philosophic mind, ever ready to widen the scope of the horizon when a novel supposition is indubitably proved to be a real fact, and to assimilate that truth into the theory, widening correspondingly the scope of the latter if needful, is obviously the ideal thing to cultivate, and one which eventually finds itself in harmony with the authenticated final results of the new discoveries. It usually happens that too sweeping conclusions are at first drawn from such new facts, but time, with its further wealth of experience, especially the accumulation of experimental data which it brings in its train, soon levels these down and relegates the facts to their proper positions in the great scheme of natural knowledge.
Lehmann’s view is that the ordinary effect of surface tension to cause truly liquid particles to assume the spherical “drop” form is resisted by a special force, which he terms “Gestaltungskraft,” and which we may perhaps translate “Configuration-determining force.” This force he considers is not identical with that of elasticity, but is that force by virtue of which a “flowing crystal” continually seeks, while freely swimming in the mother liquor or fused liquid, to take up its normal configuration. Even if a spherical drop could be cut out of it, the sphere would at once become a rod, prism, or pyramid or whatever the normal configuration of the flowing crystals of the substance in question might be.
The much debated term “liquid crystal” has been given by Lehmann to the normal configuration of each of the now considerable number of substances which have been discovered to exhibit the phenomena of flowing crystals. The latter appellation “flowing crystal,” which Lehmann also uses, appears to the author to be in many ways more suitable, however, and would avoid much of the criticism which has been levelled at the term “liquid crystal.”
As already indicated, Lehmann attributes the whole phenomena to a fundamental cause, namely, anisotropy (optical dissimilarity in different directions) of the molecules themselves, which he considers must cause self-restoration of the structure after disturbance, a process which he terms “spontaneous homœotropy.” He considers that it is the molecular configuration-producing force, connected with the tabular form of, and directionally differentiated distribution of energy and force in, the single chemical molecules, which maintains the inner structure of the flowing crystal in position. The polyhedral outward form thus appears to be a necessary consequence of the inner structure, on this basis that it is a force resident in the molecules themselves which produces the structure.